水下桩腿长度变化对浮式结构固有频率的影响研究

The modulation effects of the legs on the natural frequencies of the floating structure

为了分析带支腿浮式结构水下桩腿处于不同的长度时振动固有频率的变化情况,采用有限元方法和三维线性水弹性理论对一理想的带支腿浮式结构进行研究,分工况计算了3种弹性模态的干、湿固有频率。结果表明对于同一阶弹性模态来说,结构主体平台振型是决定水动力系数大小的主要因素,桩腿水下长度是次要因素,而振型又受到桩腿长度的制约;桩腿水下长度的改变对带支腿浮式结构的干模态固有频率以及湿固有频率都有较明显地调制作用。对于干模态固有频率来说,桩腿关于船身越趋于对称分布时结构的共振频率越高;对于湿固有频率来说,水下桩腿长度地增加有迫使共振频率趋向低频方向移动趋势。

To analyze the dry and wet natural frequencies of the floating structure with legs,the hydrodynamic coefficients and the stiffness coefficients of the floating structure with legs are calculated by utilizing the Finite Element Method and the 3 D linear hydroelasticity theory. The results show the main factor that determines the hydrodynamic coefficients of the identical elastic mode is the mode shape of the main platform,and the length of the legs is the secondary factor,however,the mode shape of the main platform is restricted by the legs. The variation of the leg＇s length underwater can prominently modulate both the dry natural frequencies and the wetted natural frequencies of the floating structure with legs. In the dry condition,with the legs turning to the symmetrical distribution about the main body,the natural frequency of the structure tends to move to the high frequency domain. In the wet condition,the modulation effects of the legs on the natural frequencies are manifested by adjusting the added mass. With the legs stretching out from the bottom of the main body,the wetted frequencies have the tendency to move to the low frequency domain.